Polyolefin polymers have been used as components of curable insulation compositions that are extruded onto electrical conductors and cured by heat. The prior art has recognized the desirability of adding stabilizers and flame retardants to polyolefin compositions to prevent thermal degradation of the base polymer and to avoid introducing flammable insulating materials into commercial or domestic environments. For example, to be economical and practical, polyolefin insulation systems used for electrical conductors such as building wire, electric motor wire, home appliance wire, underground power transmitting cables should be durable and retain their initial physical properties.
It is well known that the normal operating temperatures encountered in the environments that electrical devices are used are sufficiently elevated that oxidative degradation can be expected to occur. This phenomenon is commonly called "thermal aging".
The use of flame retardant agents to impart flame retardant properties to polyolefins is also well known in the art. It is also known that the addition of flame retardants can adversely affect thermal stability. The debilitating oxidative deterioration of polymeric compositions, such as physical embrittlement and cracking, is a subject of vast and intensive concern and has been the subject of substantial research activity. For example, U.S. Pat. Nos. 3,819,410 and 3,879,180 are concerned with the problem of polymer oxidation and provide a number of proposed solutions. U.S. Pat. No. 4,260,661, which is incorporated by reference describes the use of a sterically hindered di-tertiary butyl phenol with at least one zinc salt of a mercaptoimidazole selected from the group consisting of a zinc salt of 2-mercaptobenzimidazole or a zinc salt of 2-mercaptotolylimidazole.